A　model　to　calculate　the　thermodynamic　functions　of　the　pure　metal　nanocrystals　has　been　developed,　with　the　consideration　of　the　effects　of　both　the　interfaces　and　the　crystal　in　the　nano-grain　interior.　As　an　example,　the　enthalpy,　entropy　and　Gibbs　free　energy,　as　functions　of　the　excess　free　volume　at　interfaces,　temperature　and　grain　size,　are　calculated　for　the　Co　nanocrystals.　Furthermore,　the　characteristics　of　β-Co&rarrα-Co　phase　transformation　are　studied,　and　the　transformation　temperatures　at　different　levels　of　grain　size,　as　well　as　the　critical　grain　sizes　at　different　temperatures,　are　predicted.　The　calculation　results　show　that,　the　nano-grained　β-Co　(fcc)　is　thermodynamically　stable　at　temperatures　much　lower　than　that　for　the　conventional　coarse-grained　materials,　and　may　also　stably　exist　at　room　temperature　when　the　grain　size　is　reduced　to　be　small　enough.　The　present　model　is　verified　by　comparisons　between　the　experimental　findings　and　the　theoretical　predictions.